1. Field of the Invention
The present invention relates to rare-earth alloys and a method of manufacturing such alloys. The invention also relates to Sm2Co17-based sintered magnets and a method of manufacturing such magnets.
2. Prior Art
The sintered magnet materials used in Sm2Co17-based permanent magnets are typically produced by a process which includes milling an alloy ingot of a regulated composition to a particle size of 1 to 10 μm, pressing and shaping the resulting powder in a magnetic field to form a powder compact, sintering the powder compact in an argon atmosphere at 1100 to 1300° C., and typically about 1200° C., for a period of 1 to 5 hours, then solution-treating the sintered compact. Next, the solution-treated compact is generally subjected to aging treatment in which it is held at a temperature of 700 to 900° C., and typically about 800° C., for about 10 hours, then gradually cooled to 400° C. or less at a rate of −1.0° C./min. In a conventional process of this type, sintering and solution treatment must be carried out under strict temperature control within an optimal range of ±3° C. about the temperature setting. The reason is that, during sintering and solution treatment, the presence of a plurality of different constituent phases gives rise to local heat treatment temperature-sensitive variations in crystal grain growth and phase transitions. Moreover, temperature control during sintering and solution treatment tends to become even more rigorous for Sm2Co17-based sintered magnets of higher magnetic properties. A uniform alloy structure that is as free of segregation as possible is essential for maintaining the treatment temperature with the optimal temperature range and achieving good magnetic properties.
One casting technique used to obtain Sm2Co17-based magnet alloys having a uniform structure involves casting an alloy melt into a mold having a box-like or other suitable shape so as to form a macroscopic structure composed of columnar crystals. In such a process, the cooling rate of the alloy melt must be increased to some degree in order to form columnar crystals. Yet, in a casting process carried out using a box-shaped mold, the inner portions of the ingot tend to cool more slowly than the cooling rate at which columnar crystals form, resulting in a larger grain size and the formation of equiaxed crystals. One way to overcome this problem is to reduce the thickness of the ingot, but doing so lowers the production efficiency. Hence, ingots having a substantial degree of thickness are generally produced, often resulting in a coarser structure and the formation of equiaxed crystals. Coarsening of the structure and equiaxed crystal formation leads to segregation within the ingot, which adversely impacts the magnet structure following sintering and solution treatment, making it difficult to achieve good magnetic properties.
One solution that has been proposed is a single-roll strip casting process (JP-A 8-260083). Ingots produced by this process have a fine crystal structure and a uniform alloy structure free of segregation. However, it has been shown that sintered magnets produced from ingots with a microcrystalline structure as the starting material, while having a better coercivity than sintered magnets made from ingots cast in a box-shaped mold, have an inferior residual flux density and maximum energy product (JP-A 9-111383). Ingots with a microcrystalline structure have a much smaller average crystal grain size than ingots cast in a box-shaped mold. When these respective types of ingots are each milled into fine powders having an average particle size of 5 μm during sintered magnet production, the average crystal grain size and the average particle size of the fine powder obtained by milling are similar for those ingots having a microcrystalline structure. Hence, the milled particles are not all single crystals; a greater proportion are polycrystalline, which lowers the degree of orientation when the powder is compacted in a magnetic field. The sintered magnet obtained after heat treatment thus has a lower degree of orientation, and ultimately a lower residual flux density and maximum energy product. For this reason, strip-cast ingots are not used as the starting material in the production of Sm2Co17-based sintered magnets.
Regardless of whether an ingot cast in a box-shaped mold or an ingot made by a strip casting process is used, the constituent phases of the Sm2Co17-based permanent magnet alloy after it has been cast are the same, and include a Th2Zn17 phase, a Th2Ni17 phase, a 1:7 phase, a 1:5 phase, a 2:7 phase and a 1:3 phase. Strict temperature control is required with the optimal temperature range during sintering and solution treatment being ±3° C.